1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a twisted nematic liquid crystal display device employing a nematic liquid crystal material having positive dielectric anisotropy.
2. Description of the Prior Art
It has been known in the art to utilize a liquid crystal material for display purposes. Particularly, a twisted nematic liquid crystal cell has often been employed in liquid crystal display devices. A liquid crystal display device of this type is provided with a liquid crystal cell which comprises a pair of oppositely disposed transparent front and rear substrates bearing on the inside surface thereof a patterned conductive electrode layer, and a nematic-phase liquid crystal material of positive dielectric anisotropy filling the space therebetween. The inside surfaces of the transparent substrates are rubbed or provided with oblique incidence deposition of inorganic material at right angles to each other so as to effect a twisted nematic structure. Adjacent the front and rear substrates are provided crossed linear polarizers to constitute a liquid crystal display assembly which transmits light therethrough when no electric field is applied across the liquid crystal cell. The transmittivity of collimated light through the twisted nematic liquid crystal display assembly between crossed linear polarizers is decreased as a function of the electric field strength applied across the liquid crystal cell. In the absence of an electric field, an observer observing the device in the direction normal to the plane of the substrates and parallel to the normal optical axis will observe that the field of view is extinguished only if the direction of vibration (direction of linear polarization) of one linear polarizer is parallel to that of the other. It will be understood by those skilled in the art that the transmittivity of collimated light through the twisted nematic liquid crystal display device is increased as a function of the electric field strength applied across the liquid crystal cell when the polarizers disposed adjacent the transparent substrates are oriented in parallel to each other.
The space between the substrates, i.e. the thickness of the liquid crystal cell, is normally 1 to 50 microns. As the nematic-phase liquid crystal, a material of positive dielectric anisotropy such as p-n-hexyl benzylidene-p'-cyanoaniline, is employed.
In the conventional twisted nematic liquid crystal display device, a pair of crossed linear polarizers are generally fixed one to either surface of a liquid crystal cell including a pair of glass substrates by means of adhesive, tackifier or the like. It is, however, very difficult and requires a great deal of skill to fix the polarizers to the surface of the substrates without forming bubbles between the polarizers and the substrates. Further, the marginal portion of the polarizers are apt to peel off during the operation of fixing the polarizers to the surface of the substrates.
The polarizers are susceptible to scratching and become weak in high humidity and heat, and accordingly, the life thereof is short in comparison with the liquid crystal cell. Therefore, it is sometimes desired to replace the polarizers with new ones. Further, it is sometimes desired to rotate one linear polarizer by 90.degree. with respect to the other linear polarizer about the optical axis of the liquid crystal cell to reverse the displayed pattern, from dark-on-light to light-on-dark image-background combination or vice versa. In addition, it is also sometimes desired to replace an achromatic linear polarizer with a colored linear polarizer to effect color display, i.e. to change dark-on-light (or light-on-dark) image-background combination to colored-on-light or dark-on-colored (or colored-on-dark or light-on-colored) combination.
In the conventional liquid crystal device as described above, however, the polarizers cannot be changed or rotated since they are fixed to the substrates by adhesives or the like.